Width is commonly used to describe the measure of an object from side to side, or, particularly in shapes and geometry, width is the shorter side of an object.
Refer to the rectangles below. Both are the same rectangle, but one is rotated just to show that in both, the shorter side of the rectangle is called the width.
There can be some confusion with the terms width and length because a width is technically a length (as is a height), but the term "length" is often used alongside width in geometric contexts to describe the longer of the lengths in an object, such as in the rectangles above.
Below is an example of an everyday object where both our definitions of width apply. The shorter measurement of the door is its width as well as its measure from side to side, as indicated by the double arrow.
To add to the confusion, since the door is standing vertically, the longer length of the door (which is a rectangle) may be referred to as its height. If you were to ask someone what the length of the door is, they might ask you "Which length?" That's why it's important to pay attention to the context in which these terms are being used. It would be clearer to ask "What are the dimensions of the door?"